Process for preparing ethylene and propylene

ABSTRACT

The present invention provides a process for preparing ethylene and/or propylene, comprising the steps of contacting a stream comprising C4+ olefins with a zeolite-comprising catalyst at a temperature in the range of from 350 to 1000° C. and retrieving an olefinic product stream comprising:
         ethylene and/or propylene, and   a C4+ hydrocarbon fraction, comprising paraffins, normal olefins and iso-olefins;       

     The C4+ hydrocarbon fraction is recycled while part of the fraction is purged. The part of the C4+ hydrocarbon with is purged is treated to extract C4+ isoolefins as tert-alkyl ethers. At least part of tert-alkyl ethers are converted to further ethylene and propylene.

This application claims the benefit of European Application No.11180320.1 filed Sep. 7, 2011, which is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a process for preparing ethylene and/orpropylene.

BACKGROUND TO THE INVENTION

Conventionally, ethylene and propylene are produced via steam crackingof paraffinic feedstocks including ethane, propane, naphtha andhydrowax. An alternative route to ethylene and propylene is anoxygenate-to-olefin (OTO) process. Interest in OTO processes forproducing ethylene and propylene is growing in view of the increasingavailability of natural gas. Methane in the natural gas can be convertedinto for instance to methanol or dimethylether (DME), both of which aresuitable feedstocks for an OTO process.

In an OTO process, an oxygenate such as methanol is provided to areaction zone comprising a suitable conversion catalyst and converted toethylene and propylene. In addition to the desired ethylene andpropylene, a substantial part of the methanol is converted to higherhydrocarbons including C4+ olefins and paraffins. In order to increasethe ethylene and propylene yield of the process, the C4+ olefins may berecycled to the reaction zone or alternatively further cracked in adedicated olefin cracking zone to produce further ethylene andpropylene. In WO2009/156433, process is described, wherein an oxygenatefeedstock is converted in an OTO zone (XTO zone) to an ethylene andpropylene product. Higher olefins, i.e. C4+ olefins, produced in the OTOzone are directed to an olefin cracking zone (OC zone). In the olefincracking zone, part of the higher olefins are converted to additionalethylene and propylene, however substantial amounts of higher olefinsremain in the effluent of the olefin cracking zone. After separating theethylene and propylene from the effluent of the olefin cracking zone,the remaining effluent of the olefin cracking zone is recycled and fedto the inlet of the olefin cracking zone together with the higherolefins stream from the OTO zone. A problem encountered with the processdescribed in WO2009/156433, is the build-up of C4+ paraffins in therecycle to the olefin cracking zone. Paraffins are produced as a sideproduct in the OTO reaction and accumulate in the bottom stream of thedepropaniser together with the C4+ olefin fraction during the work-up ofboth the OTO zone as the olefin cracking zone effluent. The C4+paraffins are not converted in the olefin cracking zone and thereforeremain in the recycle. Due to the small differences in boilingtemperature of the olefins and corresponding paraffins, the C4+paraffins are difficult to separate from the C4+ olefin fractionrecycle. To maintain acceptable levels of paraffins in the C4+ olefinfraction recycle it is therefore necessary to with draw part of the C4+olefin fraction recycle to the olefin cracking zone as a purge stream.Consequently, part of the valuable C4+ olefins are lost as part of thepurge stream.

In U.S. Pat. No. 6,049,017, a similar process is described, wherein anoxygenate feedstock is converted in an OTO zone over a SAPO-34 catalystto an effluent comprising ethylene, propylene, butylenes and paraffins.A stream comprising butylenes and paraffins is separated from the OTOzone effluent and directed to a butylenes cracking zone, whereinbutylenes are cracked over a SAPO-34 catalyst. The cracked effluent ofthe butylenes cracking zone is combined with the effluent of the OTOzone, thereby allowing any remaining butylenes in the cracked effluentto be recycled to the butylenes cracking zone. Also in the process ofU.S. Pat. No. 6,049,017, a paraffin content is built-up in the butylenesrecycle. Therefore, a purge stream (drag stream) is withdrawn from theprocess prior to feeding the stream comprising butylenes and paraffinsto the butylenes cracking zone. To prevent the loss of butenes in thepurge stream, the purge stream is provided to an oligomerisationreactor, wherein n-butenes undergo oligomerisation to higher olefins,i.e. C8, C12, c16 and higher olefins. These higher olefins aresubsequently separated from the paraffins in the purge stream anddirected to the butylenes cracking zone. A disadvantage of the processof U.S. Pat. No. 6,049,017 is that the cracking of the higher olefinsoligomerisation product results in the increased formation of paraffinsand aromatics in the butylenes cracking zone. In addition, the crackingof higher olefins in a butylenes cracking zone is much more prone tocoke formation resulting in increased catalyst deactivation.Furthermore, the oligomerisation only provides capture of n-butylenes inthe purge stream, iso-olefins are not captured, but purged from theprocess. Consequently, a less than optimal ethylene and propylene yieldis achieved.

SUMMARY OF THE INVENTION

There is a need in the art for a process allowing for an improvedutilization of C4+ olefins in a process for producing ethylene andpropylene.

It has now been found that by reacting iso-olefins in purge stream of aprocess for producing ethylene and propylene with methanol into atert-alkyl ether, the utilization of C4+ olefins can be improved byconverting the tert-alkyl ether into further ethylene and propylene.

Accordingly, the present invention provides process for preparingethylene and/or propylene, comprising the steps of

a) contacting a stream comprising C4+ olefins with a zeolite-comprisingcatalyst at a temperature in the range of from 350 to 1000° C. andretrieving an olefinic product stream comprising:

ethylene and/or propylene, and

a C4+ hydrocarbon fraction, comprising paraffins, normal olefins andiso-olefins;

b) recycling a first part of the C4+ hydrocarbon fraction to step (a).

c) subjecting a second part of the C4+ hydrocarbon fraction to anetherification process with methanol and/or ethanol wherein at leastpart of the iso-olefins are converted with methanol and/or ethanol to antert-alkyl ether, and retrieving an etherification product stream;d) separating at least part of the etherification product stream into atleast an ether-enriched stream and an iso-olefin-depleted C4+hydrocarbon stream;e) withdrawing at least part of the iso-olefin-depleted C4+ hydrocarbonstream from the process to purge part of the paraffinic C4+hydrocarbons;f) converting at least part of the tert-alkyl ether in theether-enriched stream to ethylene and/or propylene by contacting atleast part of the ether-enriched stream with a molecularsieve-comprising catalyst at a temperature in the range of from 350 to1000° C.

By extracting iso-olefins from the purge stream of a process to produceethylene and propylene in the form of tert-alkyl ether, additionalethylene and/or propylene may be produced either by recycling thetert-alkyl ether back to step (a) of the process to produce ethylene andpropylene together with the remaining C4+ olefins or by separatelyconverting the tert-alkyl ether to ethylene and propylene in for examplean oxygenate-to-olefins (OTO) process.

An advantage of extracting the iso-olefins from the purge stream and notfrom the entire C4+ hydrocarbon fraction is that a much smalleretherification unit is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic representation of a process according to theinvention.

FIG. 2 provides a schematic representation of another process accordingto the invention.

FIG. 3 provides a schematic representation of a further processaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Ethylene and propylene can be produced from C4+ olefins, in particularC4 and C5 olefins, by contacting a stream comprising C4+ olefins with azeolite-comprising catalyst at elevated temperatures. The effluent ofsuch a conversion process typically comprises the desired ethyleneand/or propylene, but also comprises a C4+ hydrocarbon fraction, whichcomprises further olefins, either non reacted or newly formed, andparaffins. This C4+ hydrocarbon fraction is typically recycled to bepart of the feed to the conversion process. The paraffins in the C4+hydrocarbon fraction are also recycled, but are not converted.Consequently, paraffin levels in the feed to the process increase as C4+hydrocarbon fraction is continuously recycled. In order to maintainacceptable levels of paraffins in the feed to the process, part of theC4+ hydrocarbon fraction is withdrawn from the process as a purgestream. As it is difficult to separate olefins from their correspondingparaffins, inevitably also part of the valuable olefins are lost whenpurging part of the C4+ fraction.

In the process according to the present invention, measures are providedto reduce the amount of valuable olefins that are lost with the paraffinpurge. In the process according to the invention, iso-olefins areextracted from the purge stream, prior to withdrawing the purge streamfrom the process. The iso-olefins are extracted by reacting theiso-olefins with methanol and/or ethanol, but preferably methanol, toform tert-alkyl ether, such as for example methyl tert-butyl ether(MTBE) or tert-amyl methyl ether (TAME). The formed ethers can beseparated from the remainder of the C4+ fraction that is to be purged.Only iso-olefins, wherein the double bound is located directly adjacentto a tertiary carbon atom can react with methanol to form tert-alkylethers. Such iso-olefins are herein referred to as tertiary iso-olefins.Examples of such tertiary iso-olefins include isobutene,2-methyl-1-butene and 2-methyl-2-butene. An example of an iso-olefinthat is not a tertiary iso-olefin is 3-methyl-1-butene. Therefore, inthe process according to the present invention at least part of theiso-olefins in the C4+ hydrocarbon fraction should be tertiaryiso-olefins. By extracting the iso-olefins from the C4+ fraction to bepurged, the olefin concentration in the purge stream fraction islowered, while the paraffin concentration is increased.

The tert-alkyl ethers, obtained by extracting the iso-olefins from theC4+ hydrocarbon fraction by reacting the iso-olefins with methanol, maybe used to produce further ethylene and/or propylene. As a result lessolefins are lost with the paraffins in the paraffin purge.

The process according to the invention is now described in more detailherein below.

The process according to the present invention is a process forpreparing ethylene and/or propylene. In the process according to theinvention, a stream comprising C4+ olefins is provided. Preferably, thestream comprising C4+ olefins comprises at least C4 and/or C5 olefins,preferably at least C4 olefins. More preferably, the stream comprisingC4+ olefins comprises in the range of from 10 to 100 wt % of C4 and/orC5 olefins based on the weight of the olefins in the stream comprisingC4+ olefins, preferably of from 50 to 100 wt % of C4 and/or C5 olefinsbased on the weight of the olefins in the stream comprising C4+ olefins.Even more preferably, the stream comprising C4+ olefins comprises in therange of from 10 to 100 wt % of C4 olefins based on the weight of theolefins in the stream comprising C4+ olefins, preferably of from 50 to100 wt % of C4 olefins based on the weight of the olefins in the streamcomprising C4+ olefins. Optionally, the stream comprising C4+ olefinsalso contains a diluent. Examples of suitable diluents include, but arenot limited to, water or steam, nitrogen, argon and methane.

The stream comprising C4+ olefins is contacted with a zeolite-comprisingcatalyst. Reference herein to a zeolite comprising catalyst is to acatalyst comprising at least a zeolite. The stream comprising C4+olefins may contacted with the zeolite-comprising catalyst in a reactor.

The stream comprising C4+ olefins is contacted with thezeolite-comprising catalyst at a temperature of 350 to 1000° C.,preferably from 350 to 750° C., more preferably 450 to 700° C., evenmore preferably 500 to 650° C.; and a pressure from 0.1 kPa (1 mbar) to5 MPa (50 bar), preferably from 100 kPa (1 bar) to 1.5 MPa (15 bar).

When C4+ olefins, and in particular C4 and/or C5 olefins, are contactedwith zeolites, i.e. the zeolite in the zeolite-comprising catalyst, atthe prescribed temperatures the C4+ olefins are converted to at leastethylene and/or propylene, preferably ethylene and propylene. Inaddition to ethylene and/or propylene, also paraffins are formed,including at least C4+ paraffins.

Following the contacting of the stream comprising C4+ olefins with thecatalyst an olefinic product stream can be retrieved from the reactor.The olefinic product stream comprises at least ethylene and/orpropylene. Preferably, the olefinic product comprises advantageously atleast 50 mol %, in particular at least 50 wt %, ethylene and propylene,based on total hydrocarbon content in the olefinic product. In addition,the olefinic product stream comprises a C4+ hydrocarbon fraction.Reference herein to a C4+ hydrocarbon is to hydrocarbons comprising 4 ormore carbon atoms. Preferably, the C4+ hydrocarbon fraction comprises inthe range of from 80 to 100 wt % of C4+ hydrocarbons, more preferably of90 to 100 wt % of C4+ hydrocarbons, based on the total weight of the C4+hydrocarbon fraction. This C4+ hydrocarbon fraction comprises paraffins,normal olefins and iso-olefins, optionally the C4+ hydrocarbon fractionalso comprises diolefins. Each of the paraffins, normal olefins andiso-olefins may be unreacted paraffins, normal olefins and iso-olefinsoriginally contained in the stream comprising C4+ olefins or may beparaffins, normal olefins and iso-olefins formed in contact with thezeolite-comprising catalyst.

Preferably, the C4+ hydrocarbon fraction comprises at least C4 and/or C5olefins, preferably at least C4 olefins. More preferably, the C4+hydrocarbon fraction comprises in the range of from 10 to 100 wt % of C4and/or C5 olefins based on the weight of the olefins in the C4+hydrocarbon fraction, preferably of from 50 to 100 wt % of C4 and/or C5olefins based on the weight of the olefins in the C4+ hydrocarbonfraction. Even more preferably, the C4+ hydrocarbon fraction comprisesin the range of from 10 to 100 wt % of C4 olefins based on the weight ofthe olefins in the C4+ hydrocarbon fraction, preferably of from 50 to100 wt % of C4 olefins based on the weight of the olefins in the C4+hydrocarbon fraction. Preferably, the C4+ hydrocarbon fraction comprisesin the range of from 0 to 10 wt % of C6+ olefins, i.e. olefinscomprising 6 carbon atoms or more, more preferably 0 to 5 wt % of C6+olefins, based on the weight of the olefins in the C4+ hydrocarbonfraction. Preferably, in the range of from 1 to 60 wt % of the olefinsin the C4+ hydrocarbon fraction are iso-olefins, more preferably of from5 to 50 wt % of the olefins in the C4+ hydrocarbon fraction areiso-olefins, based on the weight of the olefins in the C4+ hydrocarbonfraction.

Preferably, the C4+ hydrocarbon fraction comprises in the range of from1 to 60 wt %, more preferably of from 5 to 50 wt %, of paraffins, basedon the weight of the hydrocarbons in the in C4+ hydrocarbon fraction.

The olefinic product stream may also comprise water or steam, methane,ethane, propane, C2 and/or C3 diolefins and any diluents as present inthe stream comprising C4+ olefins.

Preferably, the C4+ hydrocarbon fraction of the olefinic product streamis separated from the remainder of the olefinic product stream prior tobeing subjected to the etherification process. The C4+ hydrocarbonfraction of the olefinic product stream may separated from the remainderof the olefinic product by any suitable work-up section. The olefinicproduct stream may be separated in a fraction comprising ethylene and/orpropylene and C4+ hydrocarbon fraction. The design of the work-upsection depends on the exact composition of the olefinic product stream,and may include several separation steps.

In the process according to the present invention, a first part of theC4+ hydrocarbon fraction is recycled to step (a) of the process to formpart of the C4+ olefin-comprising stream.

To prevent a built-up of paraffins in the process, by continuouslyrecycling unreacted paraffins in the C4+ hydrocarbon fraction, at leastpart of the C4+ hydrocarbon fraction is withdrawn from the process topurge part of the paraffinic C4+ hydrocarbons in the C4+ hydrocarbonfraction. Preferably, in the range of from 1 to 5 wt % of the C4+hydrocarbon fraction is removed from the process as purge stream, thispart of the C4+ hydrocarbon fraction is also referred to as the secondpart of the C4+ hydrocarbon fraction. In the process according to theinvention, prior to withdrawing the second part of the C4+ hydrocarbonfraction from the process, the second part of the C4+ hydrocarbonfraction is first subjected to an etherification process.

In the etherification process the second part of the C4+ hydrocarbonfraction is contacted with methanol in the presence of a suitableetherification catalyst. When the iso-olefins in the C4+ hydrocarbonfraction are contacted with methanol in the presence of anetherification catalyst, at least part of the iso-olefins are convertedwith methanol to tert-alkyl ethers. Reference herein in to a tert-alkylether is to an ether of methanol and an iso-olefin. Examples of suchtert-alkyl ethers are MTBE and TAME, which are tert-alkyl ethers ofmethanol and respectively isobutene and isopentene. From theetherification process, an etherification product stream is retrieved.The etherification product stream will comprise the formed tert-alkylethers and the remainder of the second part of the C4+ hydrocarbonfraction, i.e. the unreacted components. In addition, the etherificationproduct stream may also comprise unreacted methanol.

At least part, and preferably all, of the etherification product streamis separated into at least an ether-enriched stream and aniso-olefin-depleted C4+ hydrocarbon stream. The separation of theetherification product stream into an ether-enriched stream and aniso-olefin-depleted C4+ hydrocarbon stream can be done with normalseparation means provided in the art. Typically the etherificationreaction is performed in the presence of an excess of alcohol, i.e.above reaction stoichiometry with the iso-olefin. Due to the largedifference in boiling temperature between the formed ethers and theremaining components in the etherification product stream, theetherification product stream may be separated using conventionaldistillation columns and gas/liquid separators. It is preferred toconcentrate any methanol in the ether-enriched stream. Due to therelatively high boiling point of methanol, the bulk of the excessalcohol can be directed toward the ether-enriched stream. Methanol mayform an azeotropic mixture with the C4 olefins in theiso-olefin-depleted C4+ hydrocarbon stream. The methanol concentrationin the azeotropic mixture is approximately 4 wt %, based on weight ofthe azeotropic mixture. It may be desired to remove the methanol priorto purging any part of the iso-olefin-depleted C4+ hydrocarbon stream,as methanol is a valuable feedstock for producing ethylene andpropylene. Methanol may be extracted from the iso-olefin-depleted C4+hydrocarbon stream by a water extraction. In one embodiment methanol isseparated from hydrocarbons in an extraction column. Methanol andhydrocarbons are fed to the bottom part of the extractor and water tothe top section. The column is typically filled with random packing orsieve trays, which enhance methanol mass-transfer from the hydrocarbonphase to the water phase. Essentially methanol-free hydrocarbons may beretrieved above the water feed point, and a water/methanol mixture isthe bottom product. The methanol may separated from the water bydistillation an led back to the etherification process, or, preferablythe water/methanol mixture may be contacted with a molecular sieve toproduce ethylene and/or propylene, for instance by providing thewater/methanol mixture to an OTO unit.

The iso-olefin-depleted C4+ hydrocarbon stream is withdrawn from theprocess to purge part of the paraffinic C4+ hydrocarbons.

In the process according to the present invention, the ethylene and/orpropylene yield is further increased by converting at least part of thetert-alkyl ethers in the ether-enriched stream to ethylene and/orpropylene. At least part of the tert-alkyl ethers in the ether-enrichedstream are converted by providing the ether-enriched stream to a reactorand contacting at least part of the ether-enriched stream with amolecular sieve-comprising catalyst to obtain a further olefinicproduct, comprising ethylene and/or propylene. In addition, the furtherolefinic product may also comprise a C4+ hydrocarbon fraction, which maycomprise C4+ olefins. The ether-enriched stream is contacted with themolecular sieve-comprising catalyst at a temperature in the range offrom 350 to 1000° C., preferably of from 350 to 750° C. When thetert-alkyl ethers, and in particular MTBE and/or TAME, are contactedwith molecular sieves, i.e. the molecular sieve in the molecularsieve-comprising catalyst, the tert-alkyl ethers are at least partiallyconverted to at least ethylene and/or propylene, preferably ethylene andpropylene. In addition to ethylene and/or propylene, also C4+ olefinsmay be formed. As the tert-alkyl ethers are oxygenates, the conversionof the tert-alkyl ethers in the ether-enriched stream may be consideredas an OTO process and operated as such an OTO process. Processconditions for operating an OTO process are provided herein below.

The conversion of oxygenates, such as methanol and DME, under suchconditions to olefins in the presence of molecular sieve-comprisingcatalysts is well known in the art. With respect to the tert-alkylethers it is believed, without wishing to be bound to a particulartheory, that upon contacting the molecular sieve-catalyst, thetert-alkyl ether decomposes into its corresponding alcohol, i.e.methanol, and iso-olefin, e.g. isobutene or isopentene. Thisdecomposition reaction is acid-catalysed. Therefore, preferably themolecular sieve-comprising catalyst comprises acid groups. Somemolecular sieves are acidic by nature, whereas other molecularsieve-comprising catalysts comprise binder, support, matrix or othermaterials comprising acid groups. Even theoretically non-acidicmolecular sieves typically comprise some residual acid groups introducedduring preparation of the molecular sieve and/or molecularsieve-comprising catalyst. In the absence of any acid groups in themolecular sieve-comprising catalyst it may be preferred to add suchgroups either by treating the molecular sieve-comprising catalyst tointroduce such groups essentially at the surface of the catalyst throughimpregnation with an acid that resides on the catalyst aftercalcination, for instance by treating the molecular sieve-comprisingcatalyst with an acid, such as phosphoric acid, or adding an acidcomponent to catalyst formulation comprising the molecularsieve-comprising catalyst, such as alumina.

Alternatively, the oxygenate-comprising feedstock is contacted with anacid catalyst, prior to contacting the molecular sieve-comprisingcatalyst. This may for instance be done by passing oxygenate-comprisingfeedstock through an acid catalyst comprising bed or by passing thefeedstock through an acid grid or filter. Preferably, theoxygenate-comprising feedstock is contacted with the acid catalyst at atemperature above 150° C. More preferably, the oxygenate-comprisingfeedstock is contacted with the acid catalyst at a temperature above350° C.

Preferably, steam is present as the tert-alkyl ether contacts thecatalyst. Steam is believed to increase the selectivity of the reaction.

At least part of the methanol obtained following the decomposition ofthe tert-alkyl ether is subsequently converted to ethylene and/orpropylene over the molecular sieve-comprising catalyst under the processconditions applied. Any residual methanol in the ether-enriched streamis also converted under these conditions.

As mentioned hereinabove it is believed that upon contact with themolecular sieve-comprising catalyst, the tert-alkyl ether decomposesinto methanol and an iso-olefin, e.g. isobutene or isopentene. Dependingon the nature of the molecular sieve in the molecular sieve-comprisingcatalyst, the obtained iso-olefins are either, at least partially,converted to ethylene and/or propylene or remain unconverted.

In one preferred embodiment, at least part of the tert-alkyl ether inthe ether-enriched stream is provided to step (a). In this embodiment,at least part of the ether-enriched stream is recycled to step (a),either as part of the stream comprising C4+ olefins or together with thestream comprising C4+ olefins. By recycling at least part of theether-enriched stream to step (a), steps (a) and (f) of the processaccording to the invention are at least partly combined. By contactingthe tert-alkyl ether with the zeolite-comprising catalyst in step (a),at least part of the ether is converted in to ethylene and propylenetogether with the C4+ olefins in the C4+ olefins-comprising stream. Thisembodiment has the advantage that only a single catalytic stage isrequired, although more catalytic stages may be provided, as both theconversion of the C4+ olefins and the tert-alkyl ethers is done by thesame catalyst. In case, both a C4+ olefins comprising stream and anoxygenate, such as an tert-alkyl ether, are provided to step (a), step(a) may be considered as an OTO process and operated as such an OTOprocess. Process conditions for operating an OTO process are providedherein below. Preferably, in addition to the stream comprising C4+olefins and tert-alkyl ethers, further oxygenates are provided to step(a) of the process. These further oxygenates may be provided as part ofthe stream comprising C4+ olefins, as part of the ether-enriched streamand/or separately. Preferred oxygenates to be additionally provided tostep (a) include, methanol and dimethylether. In case furtheroxygenates, such as methanol or dimethylether, are provided to the step(a), the C4+ olefins in the stream comprising C4+ olefins may beprovided solely by the recycle of C4+ olefins according to step (b) ofthe process, i.e. no external C4+ olefins are provided to the process.Sufficient C4+ olefins may be produced by step (a) of the process tomaintain a constant recycle of C4+ olefins. However, in such a case itmay be desired to provide some external C4+ olefins during start-up ofthe process.

In another preferred embodiment, the ether-enriched stream is providedto a separate OTO process and converted to ethylene and/or propylene bycontact with a molecular sieve catalyst at a temperature in the range offrom 350 to 1000°, preferably 350 to 750° C. This embodiment has theadvantage that a greater degree of catalyst choice is obtained toachieve optimal conversion of the C4+ olefins in the stream comprisingC4+ olefins and the tert-alkyl ethers, separately. In addition, thisembodiment allows for the use of an existing OTO process to convert thetert-alkyl ethers, preferably in combination with other oxygenates, suchas methanol or dimethylether. In such an embodiment, it is preferredthat at least part of any C4+ olefins in the product stream of the OTOprocess are provided to step (a) of the process according to theinvention as part of the stream comprising C4+ olefins. Step (a) of thepresent process may in such an embodiment act as an olefin crackingprocess, also referred to as OCP, used to convert at least part of theC4+ olefins in the effluent of an OTO process. Combinations of OTOprocesses with olefin cracking processes are well known in the art.Preferably in such a combination, (i) an oxygenate-comprising feedstockis provided to an oxygenate-to-olefin process to produce a productstream comprising ethylene and/or propylene and C4+ olefins, (ii) atleast part of such C4+ olefins are provided to step (a) of the processaccording to the invention, as part of the stream comprising C4+olefins, and (iii) wherein least part of the ether-enriched streamobtained in step (d) is provided to the oxygenate-to-olefin processtogether with or as part of the oxygenate-comprising feedstock. Thisembodiment is particularly preferred when the molecular sieve-comprisingcatalyst comprises at least one SAPO, AlPO, or MeAlPO type molecularsieve, preferably SAPO-34, as these catalysts are less suitable toconvert iso-olefins.

In the process according to the invention, the stream comprising C4+olefins may be any stream comprising C4+ olefins. C4+ olefins in thestream comprising C4+ olefins may be provided solely by the internalrecycle of C4+ olefins as described herein above. The stream comprisingC4+ olefins may also be an external stream. Examples of such streams mayinclude streams comprising C4+ olefins obtained from an FCC or steamcracking process. Preferably, at least part of the stream comprising C4+olefins is obtained from an OTO process, wherein an oxygenate-comprisingfeed is converted to a product stream comprising ethylene and/orpropylene and comprising C4+ olefins. Preferably, theoxygenate-comprising feed is converted by contacting the feed with amolecular-sieve-comprising catalyst at a temperature in the range offrom 350 to 1000° C., more preferably 350 to 700° C.

At least part of the stream comprising C4+ olefins can be obtained byseparating a fraction comprising C4+ olefins from the streams obtainedfrom e.g. an FCC, steam cracking or OTO process. This fractioncomprising C4+ olefins may separated from the remainder these streams byany suitable work-up section. The design of the work-up section dependson the exact composition of the streams, and may include severalseparation steps.

It is particularly preferred that at least part of the stream comprisingC4+ olefins is obtained from an OTO process, of which theoxygenate-comprising feed to that OTO process comprised at least partether-enriched stream. As described herein above, feeding the tert-alkylethers obtained in step (c) of the process to a process wherein they arecontacted with a molecular sieve-comprising catalyst at elevatedtemperatures may result a decomposition of the tert-alkyl ether into atleast an iso-olefin, such as iso-butene and isopentene. Any unconvertediso-butene and isopentene may preferably be provided to step (a) of theprocess as part of the stream comprising C4+ olefins, wherein at leastpart of these iso-olefins are converted to ethylene and/or propylene incontact with the zeolite-comprising catalyst of step (a).

Optionally, prior to converting the ether-enriched stream to furtherethylene and/or propylene, at least part of the tert-alkyl ether in theether-enriched stream is decomposed in to methanol and an iso-olefin,and subsequently at least part of the iso-olefin is isomerised to anormal-olefin. This has the advantage that normal-olefins are moreeasily converted to at least ethylene and/or propylene than iso-olefins,when contacted with the molecular sieve-comprising catalyst in an OTOprocess. Decomposition of the tert-alkyl ether can be done by a processand catalyst as used for the original etherification, however by using atemperature above 150° C., preferably above 175° C., more preferablyabove 200° C., the etherification reaction is reversed.

In addition to catalyzing the decomposition of the ether, theetherification catalyst may also induce skeletal isomerisation of theresulting iso-olefin to its corresponding normal-olefin at the mentionedtemperatures, although temperatures in the range of from 250 to 350° C.are preferred. Alternatively, at least part of the iso-olefins isisomerised in a separate isomerisation process. Such an isomerisationprocess may be any isomerisation process known for isomerising ofiso-olefins to normal olefins. One such process, involves contacting theiso-olefins with a SAPO-11 molecular sieve at temperatures above 200° C.

As mentioned herein above, the purge stream still comprises normal C4+olefins. The yield of ethylene and/or propylene may be even furtherincreased by subjecting normal olefins in the purge stream to anisomerisation process, wherein at least part of the normal olefins areisomerised to iso-olefins. This may be done by a process similar to theprocess used to isomerise iso-olefins to normal olefins, as the reactionis an equilibrium reaction. The formed iso-olefins may be extracted fromthe purge stream for instance by providing at least part of the purgestream comprising iso-olefins back to the etherification process in step(c) of the process as part of or together with the second part of theC4+ hydrocarbon fraction. Alternatively, at least part of the purgestream comprising the newly formed iso-olefins is subjected to aseparate etherification process.

In the process according to the invention, iso-olefins are reacted withmethanol in an etherification process. The etherification process may beany suitable etherification process available in the art for etherifyingmethanol and iso-olefins to tert-alkyl ethers. Reference is made to theHandbook of MTBE and Other Gasoline Oxygenates, H. Hamid and M. A. Alied., 1^(st) edition, Marcel Dekker, New York, 2004, pages 65 to 223,where several established process and catalyst for preparing tert-alkylethers such as MTBE and TAME are described. In particular reference ismade to chapter 9, pages 203 to 220 of the Handbook of MTBE and OtherGasoline Oxygenates, wherein suitable commercial etherificationprocesses are described. A preferred etherification process is anetherification process wherein the iso-olefins are converted withmethanol to a tert-alkyl ether in the presence of a catalyst. Anyhomogeneous or heterogeneous Brönsted acid may be used to catalyze theetherification reaction. Such catalyst include: sulfuric acid, zeolites,pillared silicates, supported fluorocarbonsulphonic acid polymers andprotonated cation-exchange resins catalyst, preferred catalyst areprotonated cation-exchange resins catalyst due to the higher catalyticactivity and the bound acid sites. A commonly used catalyst is Amberlyst15.

Preferably, the iso-olefins are converted with methanol to a tert-alkylether at a temperature in the range of from 30 to 100° C., morepreferably 40 to 80° C. Preferably, the iso-olefins are converted withmethanol to a tert-alkyl ether at a pressures in the range of from 5 to25 bar, more preferably 6 to 20 bar.

The iso-olefins may be converted with methanol to a tert-alkyl ether inany etherification process, however, one preferred etherificationprocess is based on a reactive distillation, which allows for acontinuous etherification and separation of the formed ethers.

The C4+ hydrocarbon fraction may comprise diolefins. Preferably, atleast the part of the C4+ hydrocarbon fraction subjected to theetherification process is selectively hydrogenated to remove at leastpart of any diolefins, by hydrogenating the diolefins to mono-olefinsand/or paraffins, preferably to mono-olefins.

In the present invention, an oxygenate feedstock, including feedstockscomprising tert-alkyl ethers, may be converted in anoxygenate-to-olefins (OTO) process. This may be any suitable OTO processknown in the art in the art. Preferably, it is an OTO process in whichan oxygenate feedstock is contacted in an OTO zone with an oxygenateconversion catalyst under oxygenate conversion conditions, to obtain aconversion effluent comprising lower olefins. Reference herein to anoxygenate feedstock is to an oxygenate-comprising feedstock. In the OTOzone, at least part of the feedstock is converted into a productcontaining one or more olefins, preferably including lower olefins, inparticular ethylene and typically propylene. Preferably, the productcomprises advantageously at least 50 mol %, in particular at least 50 wt%, ethylene and propylene, based on total hydrocarbon content in theproduct.

The oxygenate used in the process according to the invention ispreferably an oxygenate which comprises at least one oxygen-bonded alkylgroup. The alkyl group preferably is a C1-C5 alkyl group, morepreferably C1-C4 alkyl group, i.e. comprises 1 to 5, respectively, 4carbon atoms; more preferably the alkyl group comprises 1 or 2 carbonatoms and most preferably one carbon atom. Examples of oxygenates thatcan be used in the oxygenate feedstock include alcohols and ethers.Examples of preferred oxygenates include alcohols, such as methanol,ethanol, propanol; and dialkyl ethers, such as dimethylether,diethylether, methylethylether. Preferably, the oxygenate is methanol ordimethylether, or a mixture thereof.

In the process according to the invention, it is preferred that theoxygenate feedstock also includes a tert-alkyl ether produced by step(c) of the process according to the invention.

Preferably the oxygenate feedstock comprises at least 50 wt % ofoxygenate, in particular methanol and/or dimethylether, based on thetotal of hydrocarbons and oxygenates in the oxygenate feedstock, morepreferably at least 70 wt %.

The oxygenate feedstock can comprise an amount of diluents. During theconversion of the oxygenates, steam is produced as a by-product, whichserves as an in-situ produced diluent. Optionally additional steam isadded as diluent. The amount of additional diluent that needs to beadded depends on the in-situ water make, which in turn depends on thecomposition of the oxygenate-comprising feed. Where methanol produces 1mol of water per mol of carbon atoms supplied to the process, MTBE, forexample only produces 0.20 mol of water per 1 mol of carbon atomssupplied to the process. Where the diluent is water or steam, the molarratio of oxygenate to diluent is between 10:1 and 1:20. In case, theoxygenate-comprising feedstock comprises in the range of from 0.01 to 50wt %, preferably of from 1 to 10 wt %, of tert-alkyl ether, based on theoxygenates in the oxygenate-comprising feedstock, the molar ratio ofoxygenate to diluent is preferably in the range of from 3:1 to 1:5,preferably 2:1 to 1:2. In case, the oxygenate-comprising feedstockcomprises in the range of from 50 to 100 wt %, preferably 60 to 95 wt %,of tert-alkyl ether, based on the oxygenates in the oxygenate-comprisingfeedstock, the molar ratio of oxygenate to diluent is preferably in therange of from 1:3 to 1:15, preferably 1:4 to 1:10.

Due to the low in-situ water make of tert-alkyl ethers, the use ofdiluents other than water may be preferred, in particular when thecatalyst is sensitive to hydrothermal deactivation. Other suitablediluents include inert gases such as nitrogen, but may also includeparaffins.

Preferably, in addition to the oxygenate, an olefinic co-feed isprovided along with and/or as part of the oxygenate feedstock to the OTOprocess. Reference herein to an olefinic co-feed is to anolefin-comprising co-feed. The olefinic co-feed preferably comprises C4and higher olefins, more preferably C4 and C5 olefins. Preferably, theolefinic co-feed comprises at least 25 wt %, more preferably at least 50wt %, of C4 olefins, and at least a total of 70 wt % of C4 hydrocarbonspecies.

Preferably, at least 70 wt % of the olefinic co-feed, during normaloperation, is formed by a recycle stream of a C3+ or C4+ hydrocarbonfraction from the OTO conversion effluent, preferably at least 90 wt %of olefinic co-feed, based on the whole olefinic co-feed, is formed bysuch recycle stream. In order to maximize production of ethylene andpropylene, it is desirable to maximize the recycle of C4 olefins in theeffluent of the OTO process. This can be done by recycling at least partof the C4+ hydrocarbon fraction, preferably C4-C5 hydrocarbon fraction,more preferably C4 hydrocarbon fraction, in the OTO effluent. However, acertain part thereof, such as between 1 and 5 wt %, needs to bewithdrawn as purge, since otherwise saturated hydrocarbons, inparticular C4's (butane) would build up in the process, which aresubstantially not converted under the OTO reaction conditions.

The preferred molar ratio of oxygenate in the oxygenate feedstock toolefin in the olefinic co-feed provided to the OTO conversion zonedepends on the specific oxygenate used and the number of reactiveoxygen-bonded alkyl groups therein. Preferably the molar ratio ofoxygenate to olefin in the total feed, i.e. oxygenate feedstock andolefinic co-feed, lies in the range of 20:1 to 1:10, more preferably inthe range of 18:1 to 1:5, still more preferably in the range of 15:1 to1:3, even still more preferably in the range of 12:1 to 1:3. Forpurposes of calculating the molar ratio of oxygenate to olefin in thetotal feed, the olefins provided to the process as part of thetert-alkyl ether must also be taken into account.

A variety of OTO processes is known for converting oxygenates such asfor instance methanol or dimethylether to an olefin-containing product,as already referred to above. One such process is described in WO A2006/020083. Processes integrating the production of oxygenates fromsynthesis gas and their conversion to light olefins are described inUS20070203380A1 and US20070155999A1.

In the process according to the present invention two types of catalystare used, i.e. molecular sieve-comprising catalyst andzeolite-comprising catalyst. Such catalyst compositions typically alsoinclude binder materials, matrix material and optionally fillers.Suitable matrix materials include clays, such as kaolin. Suitable bindermaterials include silica, alumina, silica-alumina, titania and zirconia,wherein silica is preferred due to its low acidity.

Molecular sieves preferably have a molecular framework of one,preferably two or more corner-sharing [TO4] tetrahedral units, morepreferably, two or more [SiO4], [AlO4] and/or [PO4] tetrahedral units.These silicon, aluminum and/or phosphorous based molecular sieves andmetal containing silicon, aluminum and/or phosphorous based molecularsieves have been described in detail in numerous publications includingfor example, U.S. Pat. No. 4,567,029. In a preferred embodiment, themolecular sieves have 8-, 10- or 12-ring structures and an average poresize in the range of from about 3 Å to 15 Å.

Suitable molecular sieves are silicoaluminophosphates (SAPO), such asSAPO-17, -18, 34, -35, -44, but also SAPO-5, -8, -11, -20, -31, -36, 37,-40, -41, -42, -47 and -56; aluminiophosphates (AlPO) and metalsubstituted (silico)aluminophosphates (MeAlPO), wherein the Me in MeAlPOrefers to a substituted metal atom, including metal selected from one ofGroup IA, IIA, IB, IIIB, IVB, VB, VIIB, VIIB, VIIIB and Lanthanide's ofthe Periodic Table of Elements, preferably Me is selected from one ofthe group consisting of Co, Cr, Cu, Fe, Ga, Ge, Mg, Mn, Ni, Sn, Ti, Znand Zr.

A particular class of molecular sieves are zeolites. Suitablezeolite-comprising catalysts include those containing a zeolite of theZSM group, in particular of the MFI type, such as ZSM-5, the MTT type,such as ZSM-23, the TON type, such as ZSM-22, the MEL type, such asZSM-11, the FER type. Other suitable zeolites are for example zeolitesof the STF-type, such as SSZ-35, the SFF type, such as SSZ-44 and theEU-2 type, such as ZSM-48. Zeolites are preferred when the feedstock tobe converted comprises olefins, e.g. in step (a).

Zeolite-comprising catalysts are known for their ability to converthigher olefins to lower olefins, in particular C4+ olefins to ethyleneand/or propylene. Particular preferred zeolite-comprising catalyst forconverting higher olefins to lower olefins, and in particular convertingat least part of the stream comprising C4+ olefins to the olefinicproduct in step (a), are catalysts comprising at least one zeoliteselected from MFI, MEL, TON and MTT type zeolites, more preferably atleast one of ZSM-5, ZSM-11, ZSM-22 and ZSM-23 zeolites. In addition tothe above described conversion of higher olefins to lower olefins, thezeolite-comprising catalyst have the additional advantage that thesecatalysts are also suitable for converting oxygenates to lower olefins,in particular ethylene and/or propylene. As a result, when, as describedabove, a tert-alkyl ether is contacted with such catalyst under thedescribed process conditions, the tert-alkyl ether is decomposed intomethanol or ethanol and the corresponding iso-olefin, which may both beconverted into ethylene and/or propylene. As zeolites are a type ofmolecular sieves, references herein to molecular sieve-comprisingcatalysts include zeolite-comprising catalysts.

In one preferred embodiment, the catalysts of step (a) and step (f) arethe same zeolite-comprising catalyst and at least part of the tert-alkylether is recycled to step (a), by providing at least part of theether-enriched stream to step (a).

In another preferred embodiment, the catalyst of step (f) is anon-zeolitic molecular sieve-comprising catalyst. In this embodiment,the further olefinic product obtained in step (f) by contacting theether-enriched stream with the non-zeolitic molecular sieve-comprisingcatalyst, is at least in part provided to step (a). At least part of anyC4+ olefins in the further olefinic product may be converted to at leastethylene and/or propylene in contact with the zeolite-comprisingmolecular sieve of step (a).

Preferred catalysts, for both step (a) as well as step (f), comprise amore-dimensional zeolite, in particular of the MFI type, more inparticular ZSM-5, or of the MEL type, such as zeolite ZSM-11. Suchzeolites are particularly suitable for converting olefins, includingiso-olefins, to ethylene and/or propylene. The zeolite havingmore-dimensional channels has intersecting channels in at least twodirections. So, for example, the channel structure is formed ofsubstantially parallel channels in a first direction, and substantiallyparallel channels in a second direction, wherein channels in the firstand second directions intersect. Intersections with a further channeltype are also possible. Preferably the channels in at least one of thedirections are 10-membered ring channels. A preferred MFI-type zeolitehas a Silica-to-Alumina ratio SAR of at least 60, preferably at least80. The oxygenate conversion catalyst can comprise at least 1 wt %,based on total molecular sieve in the oxygenate conversion catalyst, ofthe second molecular sieve having more-dimensional channels, preferablyat least 5 wt %, more preferably at least 8 wt %.

Particular preferred catalyst, for both step (a) as well as step (f),include catalysts comprising one or more zeolite having one-dimensional10-membered ring channels, i.e. one-dimensional 10-membered ringchannels, which are not intersected by other channels. Preferredexamples are zeolites of the MTT and/or TON type. Preferably, thecatalyst comprises at least 40 wt %, preferably at least 50% wt of suchzeolites based on total zeolites in the catalyst.

In a particularly preferred embodiment for both step (a) as well as step(f), the catalyst comprises in addition to one or more one-dimensionalzeolites having 10-membered ring channels, such as of the MTT and/or TONtype, a more-dimensional zeolite, in particular of the MFI type, more inparticular ZSM-5, or of the MEL type, such as zeolite ZSM-11. Suchfurther zeolite (molecular sieve) can have a beneficial effect on thestability of the catalyst in the course of the process and underhydrothermal conditions.

The catalyst for both step (a) as well as step (f) may comprisephosphorous as such or in a compound, i.e. phosphorous other than anyphosphorous included in the framework of the molecular sieve. It ispreferred that an MEL or MFI-type zeolites comprising catalystadditionally comprises phosphorous. The phosphorous may be introduced bypre-treating the MEL or MFI-type zeolites prior to formulating thecatalyst and/or by post-treating the formulated catalyst comprising theMEL or MFI-type zeolites. Preferably, the catalyst comprising MEL orMFI-type zeolites comprises phosphorous as such or in a compound in anelemental amount of from 0.05 to 10 wt % based on the weight of theformulated catalyst. A particularly preferred catalyst comprisesphosphor-treated MEL or MFI-type zeolites having SAR of in the range offrom 60 to 150, more preferably of from 80 to 100. An even moreparticularly preferred catalyst comprises phosphor-treated ZSM-5 havingSAR of in the range of from 60 to 150, more preferably of from 80 to100.

It is preferred that zeolites and respectively molecular sieves in thehydrogen form are used in the catalyst for both step (a) and (f), e.g.,HZSM-22, HZSM-23, and HZSM-48, HZSM-5. Preferably at least 50 wt %, morepreferably at least 90 wt %, still more preferably at least 95 wt % andmost preferably 100 wt % of the total amount of molecular sieve used isin the hydrogen form. It is well known in the art how to produce suchmolecular sieves in the hydrogen form.

The reaction conditions of the oxygenate conversion include a reactiontemperature of 350 to 1000° C., preferably from 350 to 750° C., morepreferably 450 to 700° C., even more preferably 500 to 650° C.; and apressure from 0.1 kPa (1 mbar) to 5 MPa (50 bar), preferably from 100kPa (1 bar) to 1.5 MPa (15 bar).

Typically the catalyst deactivates in the course of the process,primarily due to deposition of coke on the catalyst. The phenomenon istypically observed in both step (a) as well as in step (f) of theprocess. Conventional catalyst regeneration techniques can be employedto remove the coke. It is not necessary to remove all the coke from thecatalyst as it is believed that a small amount of residual coke mayenhance the catalyst performance and additionally, it is believed thatcomplete removal of the coke may also lead to degradation of themolecular sieve.

The catalyst particles used in the process of the present invention canhave any shape known to the skilled person to be suitable for thispurpose, for it can be present in the form of spray dried catalystparticles, spheres, tablets, rings, extrudates, etc. Extruded catalystscan be applied in various shapes, such as, cylinders and trilobes. Ifdesired, spent catalyst can be regenerated and recycled to the processof the invention. Spray-dried particles allowing use in a fluidized bedor riser reactor system are preferred. Spherical particles are normallyobtained by spray drying. Preferably the average particle size is in therange of 1-200 μm, preferably 50-100 μm.

Step (a) of the process may be operated in a fluidized bed or movingbed, e.g. a fast fluidized bed or a riser reactor system, and also in afixed bed reactor or a tubular reactor. A fluidized bed or moving bed,e.g. a fast fluidized bed or a riser reactor system are preferred.

Step (f) of the process may be operated in a fluidized bed or movingbed, e.g. a fast fluidized bed or a riser reactor system, and also in afixed bed reactor or a tubular reactor. A fluidized bed or moving bed,e.g. a fast fluidized bed or a riser reactor system are preferred.

In step (a) of the process an olefinic product stream comprisingethylene and/or propylene is retrieved. As described herein above, atleast part of the tert-alkyl ether in the ether-enriched stream isconverted in step (f) to a further olefinic product stream comprisingethylene/and propylene. The ethylene and/or propylene may be separatedfrom the remainder of the components in the olefinic product and furtherolefinic product. Preferably the olefinic product and further olefinicproduct at least partially, and preferably fully, combined prior toseparating the ethylene and/or propylene from the remaining components.Where the olefinic product comprises ethylene, least part of theethylene may be further converted into at least one of polyethylene,mono-ethylene-glycol, ethylbenzene and styrene monomer. Where theolefinic product comprises propylene, at least part of the propylene maybe further converted into at least one of polypropylene and propyleneoxide.

In the process according to the invention at least part of tert-alkylethers are converted to ethylene and/or propylene. Optionally, anotherpart of the tert-alkyl ethers are exported from the process as products,e.g. MTBE and/or TAME. Such ethers are suitably used as fuel additives.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, a process according to the present invention is schematicallyrepresented, wherein the tert-alkyl ether comprised in theether-enriched stream is converted together with the stream comprisingC4+ olefins in contact with an zeolite-comprising catalyst in reactionzone 5. In FIG. 1, stream 1 comprising C4+ olefins is provided toreactor zone 5. In reactor zone 5, stream 1 is contacted with a zeolitecomprising catalyst, for example a catalyst comprising ZSM-5, such as acatalyst comprising 50 wt % of ZSM-5 and 50 wt % ZSM-23, based on thezeolite content in the catalyst. Olefinic product stream 10 is retrievedfrom reactor zone 5 and provided to separation zone 15. Separation zone15 may for instance include a quench tower set for receiving olefinicproduct stream 10. In the quench tower high boiling components such aswater may be removed, the remaining stream may be compressed in a multistage compressor zone, with interstage cooling and separation ofcondensed phases. The compressed vapour stream may be provided to acombination of a de-ethaniser and a de-propaniser to separate thecompressed vapour stream into at least ethylene and/or propylene and aC4+ hydrocarbon fraction. The ethylene and/or propylene may be retrievedfrom separation zone 15 separately or as a mixture via one or morestreams 20. C4+ hydrocarbon fraction 25 is retrieved from separationzone 15. Part of C4+ hydrocarbon fraction 25 a is recycled to reactorzone 5. Second part of C4+ hydrocarbon fraction 25 b is provided toetherification zone 30, together with methanol 35. In etherificationzone 30, C4+ hydrocarbon fraction 25 b is contacted with methanol 35over an etherification catalyst, such as for instance a protonatedcationic-exchange resin. Etherification product 40 is retrieved frometherification zone 30 and provided to second separation zone 45,wherein etherification product 40 is separated into an ether-enrichedstream 50 and an iso-olefin-depleted stream 55. Optionally, zones 30 and45 are combined into a reactive distillation zone, wherein iso-olefinsare reacted with methanol to tert-alkyl ethers, while continuouslyseparating tert-alkyl ether from the reaction mixture. Optionally, zones30 and 45 allow for the recycle of part of the iso-olefin depletedstream in case not all of the iso-olefins are converted to tert-alkylether in a single pass process. Ether-enriched stream 50 is combinedwith stream 1, while iso-olefin-depleted stream 55 purged from theprocess. Optionally, additional oxygenate (not shown), such as methanolor dimethylether, and water is added to reactor zone 5.

In FIG. 2, a process according to the present invention is schematicallyrepresented, wherein the tert-alkyl ether comprised in theether-enriched stream is converted separately in an OTO process andreaction zone 5 is operated as an OCP zone. In FIG. 2, stream 1comprising C4+ olefins is provided to reactor zone 5. In reactor zone 5,stream 1 is contacted with a zeolite comprising catalyst, for example acatalyst comprising ZSM-5, such as a catalyst comprising 50 wt % ofZSM-5 and 50 wt % ZSM-23, based on the zeolite content in the catalyst.Olefinic product stream 10 is retrieved from reactor zone 5 and providedto separation zone 215, Separation zone 215 may be similar to separationzone 15 described herein above, however the presence of a quench toweris optional depending on the concentration of water vapour in olefinicproduct stream 10. Ethylene and/or propylene may be retrieved fromseparation zone 215 separately or as a mixture via one or more streams20. C4+ hydrocarbon fraction 25 is retrieved from separation zone 215.Part of C4+ hydrocarbon fraction 25 a is recycled to reactor zone 5.Second part of C4+ hydrocarbon fraction 25 b is provided toetherification zone 30, together with methanol 35. In etherificationzone 30, C4+ hydrocarbon fraction 25 b is contacted with methanol 35over an etherification catalyst, such as for instance a protonatedcationic-exchange resin. Etherification product 40 is retrieved frometherification zone 30 and provided to second separation zone 45,wherein etherification product 40 is separated into an ether-enrichedstream 50 and an iso-olefin-depleted stream 55. Optionally, zones 30 and45 are combined into a reactive distillation zone, wherein iso-olefinsare reacted with methanol to tert-alkyl ethers, while continuouslyseparating tert-alkyl ether from the reaction mixture. Optionally, zones30 and 45 allow for the recycle of part of the iso-olefin depletedstream in case not all of the iso-olefins are converted to tert-alkylether in a single pass process. Iso-olefin-depleted stream 55 iswithdrawn from process as purge stream. Ether-enriched stream 50 isprovided to oxygenate-to-olefin zone 200. In oxygenate-to-olefin zone200, ether-enriched stream 50 is contacted with a molecularsieve-comprising catalyst, for example a catalyst comprising ZSM-5, suchas a catalyst comprising 50 wt % of ZSM-5 and 50 wt % ZSM-23, based onthe zeolite content in the catalyst, or a catalyst comprising SAPO-34.Optionally, additional oxygenate, such as methanol or dimethylether,olefins and water are added to oxygenate-to-olefin zone 200 via conduit205. Product stream 210 is retrieved from oxygenate-to-olefin zone 200and provided to third separation zone 220, which may be a separationzone similar to separation zone 15, described hereinabove. From thirdseparation zone 220, ethylene and/or propylene may be retrievedseparately or as a mixture via one or more streams 225. C4+ hydrocarbonfraction 230 is retrieved from third separation zone 220 and provided toreaction zone 5 as stream 1.

In FIG. 3, a process according to the present invention similar to thatin FIG. 2 is schematically represented, wherein separation section 215and 220 are integrated.

In FIG. 3, a stream comprising C4+ olefins is provided to reactor zone5. In reactor zone 5, the stream comprising C4+ olefins is contactedwith a zeolite comprising catalyst, for example a catalyst comprisingZSM-5, such as a catalyst comprising 50 wt % of ZSM-5 and 50 wt %ZSM-23, based on the zeolite content in the catalyst. Olefinic productstream 10 is retrieved from reactor zone 5 and provided to separationzone 315, Separation zone 315 may be similar to separation zone 15described herein above. Olefinic product stream 10 may be provided toseparation zone 315 before or after the quench tower depending on theconcentration of water vapour in olefinic product stream 10.

An oxygenate-comprising stream is provided to oxygenate-to-olefin zone200. In oxygenate-to-olefin zone 200, the oxygenate-comprising stream iscontacted with a molecular sieve-comprising catalyst, for example acatalyst comprising ZSM-5, such as a catalyst comprising 50 wt % ofZSM-5 and 50 wt % ZSM-23, based on the zeolite content in the catalyst,or a catalyst comprising SAPO-34. Product stream 210 is retrieved fromoxygenate-to-olefin zone 200 and provided to separation zone 315,preferably such that is provided before the quench tower.

Ethylene and/or propylene may be retrieved from separation zone 315separately or as a mixture via one or more streams 20. C4+ hydrocarbonfraction 25 is retrieved from separation zone 315. Part of C4+hydrocarbon fraction 25 a is recycled to reactor zone 5. Second part ofC4+ hydrocarbon fraction 25 b is and provided to etherification zone 30,together with methanol 35. In etherification zone 30, C4+ hydrocarbonfraction 25 b is contacted with methanol 35 over an etherificationcatalyst, such as for instance a protonated cationic-exchange resin.Etherification product 40 is retrieved from etherification zone 30 andprovided to second separation zone 45, wherein etherification product 40is separated into an ether-enriched stream 50 and an iso-olefin-depletedstream 55. Optionally, zones 30 and 45 are combined into a reactivedistillation zone, wherein iso-olefins are reacted with methanol totert-alkyl ethers, while continuously separating tert-alkyl ether fromthe reaction mixture. Optionally, zones 30 and 45 allow for the recycleof part of the iso-olefin depleted stream in case not all of theiso-olefins are converted to tert-alkyl ether in a single pass process.Iso-olefin-depleted stream 55 is withdrawn from process as purge stream.Ether-enriched stream 50 is provided to oxygenate-to-olefin zone 200 as,at least part of, the oxygenate comprising stream to oxygenate-to-olefinzone 200. Optionally, additional oxygenate, such as methanol ordimethylether, olefins and water are added to form part of theoxygenate-comprising stream to oxygenate-to-olefin zone 200 via conduit205.

EXAMPLES

The invention is illustrated by the following non-limiting examples.

Example 1

Several molecular sieves were tested to show their ability to convertMTBE to an olefinic product. To test the molecular sieves for catalyticperformance, a powder of the respective molecular sieves was pressedinto tablets and the tablets were broken into pieces and sieved. MTBEwas reacted over the catalysts which were tested to determine theirselectivity towards olefins, mainly ethylene and propylene fromoxygenates. For the catalytic testing, the sieve fraction of 40-80 meshwas used. Prior to reaction, the molecular sieves were treated ex-situin air at 550° C. for 2 hours.

The reaction was performed using a quartz reactor tube of 1.8 mminternal diameter. The molecular sieve samples were heated in nitrogento the reaction temperature and a mixture consisting of 6 vol % MTBEbalanced in N₂ was passed over the catalyst at atmospheric pressure (1bar). The Gas Hourly Space Velocity (GHSV) is determined by the totalgas flow over the zeolite weight per unit time (ml·gzeolite⁻¹·h⁻¹). Thegas hourly space velocity used in the experiments was 10000(ml·gzeolite⁻¹·h⁻¹). The effluent from the reactor was analyzed by gaschromatography (GC) to determine the product composition. Thecomposition has been calculated on a weight basis of all hydrocarbonsanalyzed. The composition has been defined by the division of the massof a specific product by the sum of the masses of all products. Theeffluent from the reactor obtained at several reactor temperatures wasanalyzed. The results are shown in Table 1.

TABLE 1 T C2 = C3 = C4 = C5 Light ends C6 + C4 paraffin [° C.] Catalyst[wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] 420 SAPO-34 7.90 15.1565.43 9.18 0.19 1.06 1.09 525 SAPO-34 9.41 18.17 50.01 14.78 1.57 2.583.49 420 ZSM-5* 10.86 28.10 15.93 8.13 0.12 23.56 13.31 525 ZSM-5* 26.7738.11 11.46 2.69 0.03 13.01 7.92 525 ZSM-5^(#) 17.89 39.85 25.49 3.221.79 9.69 2.07 525 ZSM-23 20.73 42.89 29.00 2.05 0.59 3.62 1.12 525ZSM-22 17.19 39.88 35.52 2.12 0.44 3.99 0.86 *SAR 80 ^(#)SAR 280

For all tested catalyst, the conversion of MTBE was complete. No MTBE ormethanol was detected in the effluent of the reactor.

The zeolite catalysts, i.e. ZSM-5, ZSM-22 and ZSM-23, show a goodconversion of the MTBE, including the isobutene part of the MTBE, toethylene and propylene. An advantage of the one-dimensional zeoliteshaving 10-membered ring channels, i.e. ZSM-22 and ZSM-23, is the lowerparaffin make and C6+ make compared to the multi-dimensional ZSM5zeolites.

By reducing the SAR of the ZSM-5 catalyst, the ethylene and propyleneyield is improved, while significantly less C4 olefins are produced.

The non-zeolite SAPO-34 catalyst shows a low paraffin make and C6+ make,however is less suitable for converting iso-C4 olefins as can be seenfrom the relative high C4 olefin content in the effluent of the reactor.These C4 olefins are preferably subsequently converted in an OCP reactorover a zeolite catalyst. It will be clear from table 2, that zeolitecatalyst show a better conversion of C4 olefins to the desired ethyleneand propylene products. Increasing the reaction temperature, results ina reduction of the C4 olefin content in the effluent of the reaction.

Example 2

Several molecular sieves were tested to show their ability to convert amixture of MTBE and methanol to an olefinic product. To test themolecular sieves for catalytic performance, a powder of the respectivemolecular sieves was pressed into tablets and the tablets were brokeninto pieces and sieved. A mixture of MTBE and methanol was reacted overthe catalysts which were tested to determine their selectivity towardsolefins, mainly ethylene and propylene from oxygenates. For thecatalytic testing, the sieve fraction of 40-80 mesh was used. Prior toreaction, the molecular sieves were treated ex-situ in air at 550° C.for 2 hours.

The reaction was performed using a quartz reactor tube of 1.8 mminternal diameter. The molecular sieve samples were heated in nitrogento 525° C. and a mixture consisting of 3 vol % MTBE and 3 vol %methanol, balanced in N₂ was passed over the catalyst at atmosphericpressure (1 bar). The Gas Hourly Space Velocity (GHSV) is determined bythe total gas flow over the zeolite weight per unit time(ml·gzeolite⁻¹·h⁻¹). The gas hourly space velocity used in theexperiments was 10000 (ml·gzeolite⁻¹·h⁻¹). The effluent from the reactorwas analyzed by gas chromatography (GC) to determine the productcomposition. The composition has been calculated on a weight basis ofall hydrocarbons analyzed. The composition has been defined by thedivision of the mass of a specific product by the sum of the masses ofall products. The results are shown in Table 2.

TABLE 2 T C2 = C3 = C4 = C5 Light ends C6 + C4 paraffin [° C.] Catalyst[wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] 525 SAPO-34 18.11 22.0844.94 8.23 2.94 1.64 2.05 525 ZSM-5* 25.72 37.64 11.57 3.24 0.65 13.797.41 525 ZSM-5^(#) 17.66 42.42 20.31 3.31 1.82 12.88 1.61 525 ZSM-2321.45 46.66 21.09 2.77 0.81 6.16 1.06 525 ZSM-22 17.84 48.46 24.30 2.610.83 5.24 0.71 *SAR 80 ^(#)SAR 280

The zeolite catalysts do not show a significant change in the obtainedC2 to C4 olefinic product slate, when methanol is added to the MTBEfeed. As a result, it can be expected that for an existing methanolbased OTO process using a zeolite catalyst, MTBE can be blended into themethanol feed without requiring significant changes to the processoperation. In case of the SAPO-34 catalyst, the ratio of propylene toethylene obtained when using only MTBE as a feed is higher than theratio obtained from a feed comprising a mixture of MTBE and methanol. Asa result it can be concluded that blending MTBE into a methanolfeedstock to a SAPO-34 based OTO process may result in an improved ratioof propylene to ethylene without requiring significant changes to theprocess operation.

What is claimed is:
 1. A process for preparing ethylene and/orpropylene, comprising the steps of: a) contacting a stream comprisingC4+ olefins with a zeolite-comprising catalyst at a temperature in therange of from 350 to 1000° C. and retrieving an olefinic product streamcomprising: ethylene and/or propylene, and a C4+ hydrocarbon fraction,comprising paraffins, normal olefins and iso-olefins; a1) dividing theC4+ hydrocarbon fraction into a first part and a second part; b)recycling a first part of the C4+ hydrocarbon fraction to step (a); c)subjecting a second part of the C4+ hydrocarbon fraction to anetherification process with methanol and/or ethanol wherein at leastpart of the iso-olefins are converted with methanol and/or ethanol to antert-alkyl ether, and retrieving an etherification product stream; d)separating at least part of the etherification product stream into atleast an ether-enriched stream and an iso-olefin-depleted C4+hydrocarbon stream; e) withdrawing at least part of theiso-olefin-depleted C4+ hydrocarbon stream from the process to purgepart of the paraffinic C4+ hydrocarbons; f) converting at least part ofthe tert-alkyl ether in the ether-enriched stream to ethylene and/orpropylene by contacting at least part of the ether-enriched stream witha molecular sieve-comprising catalyst at a temperature in the range offrom 350 to 1000° C.
 2. A process according to claim 1, wherein step (f)comprises recycling at least part of the ether-enriched stream to step(a).
 3. A process according to claim 1, further comprising providing anoxygenate-comprising stream to step (a) and contacting theoxygenate-comprising stream with the zeolite-comprising catalysttogether with the C4+ olefins.
 4. A process according to claim 1,wherein part of the stream comprising C4+ olefins is obtained byconverting an oxygenate-comprising feed to a product stream comprising:ethylene and/or propylene, and —C4+ olefins.
 5. A process according toclaim 4, wherein the oxygenate-comprising feed is converted bycontacting the oxygenate-comprising feed with a molecularsieve-comprising catalyst at a temperature in the range of from 350 to1000° C.
 6. A process according to claim 5, wherein theoxygenate-comprising feed comprises at least part of the ether-enrichedstream.
 7. A process according to claim 4, wherein the molecularsieve-comprising catalyst comprises at least one SAPO, AlPO, or MeAlPOtype molecular sieve.
 8. A process according to claim 1, wherein thezeolite-comprising catalyst comprises at least one zeolite selected fromMFI, MEL, TON and MTT type zeolites.
 9. A process according to claim 1,wherein the iso-olefins include at least one of isobutene andisopentene.
 10. A process according to claim 1, wherein the iso-olefinsare converted with methanol to the tert-alkyl ether by contacting theiso-olefin with methanol in the presence of an etherification catalystat a temperature in the range of from 30 to 100° C.
 11. A processaccording to claim 10, wherein the etherification catalyst is aprotonated cation-exchange resin catalyst.
 12. A process according toclaim 1, wherein an oxygenate-comprising feedstock is provided to anoxygenate-to-olefin process to produce a product stream comprisingethylene and/or propylene and C4+ olefins, at least part of which C4+olefins are provided to step (a) of the process according to any one ofclaims 1 to 11, as part of the stream comprising C4+ olefins and whereinleast part of the ether-enriched stream obtained in step (e) is providedto the oxygenate-to-olefin process together with or as part of theoxygenate-comprising feedstock.
 13. A process according to claim 1,wherein the olefinic product comprises ethylene and at least part of theethylene is further converted into at least one of polyethylene,mono-ethylene-glycol, ethylbenzene and styrene monomer.
 14. A processaccording to claim 1, wherein the olefinic product comprises propyleneand at least part of the propylene is further converted into at leastone of polypropylene and propylene oxide.